Most instances of breast cancer happen later in life—often after menopause. In many cases, the cancer progresses slowly, over a period of months or even years, often giving physicians precious time to implement a treatment plan, successfully battling that cancer into remission.

A section from a mammary ‘outgrowth’ harvested at lactation. [Credit: UC San Diego School of Medicine]

But there is another far more aggressive form of breast cancer that tends to develop earlier, often immediately following pregnancy. And now, researchers at the University of California, San Diego (UCSD) have discovered how this form of cancer hijacks a woman’s own stem cells to grow quickly and spread throughout the body.

Reporting in the latest issue of the journal Developmental Cell, UCSD stem cell researchers Drs. David Cheresh and Jay Desgrosellier and their teams have found a link between the molecular signaling switches that spur this aggressive, post-pregnancy breast cancer—and mammary stem cells that are normally activated during pregnancy.

These findings, say Cheresh, offer key insight into how scientists may develop better treatments for this form of breast cancer. As he stated in a news release:

“By understanding a fundamental mechanism of mammary gland development during pregnancy, we have gained a rare insight into how aggressive breast cancer might be treated.”

Normally, pregnancy activates a special group of stem cells in the mammary gland. Their job is to ready the expectant mother for feeding the newborn baby. By the time the baby is born, however, these stem cells go back into hibernation.

However, in some women, these mammary stem cells get hijacked by cancer cells. Rather than the mammary stem cells shutting down by the time milk production begins, cancer cells keep them switched on—which then contributes to the progression of cancer.

These findings shed much-needed light on the complex relationship between breast cancer and pregnancy. However, the authors caution that these findings don’t imply that becoming pregnant causes breast cancer. Rather, as Cheresh explained:

“Our work doesn’t speak to the actual cause of cancer. Rather, it explains what can happen once cancer has been initiated.”

Cheresh, who has received CIRM support for related work, has pinpointed a protein called CD61 that may promote the progression of breast cancer. CD61 has already been implicated in cancer metastasis and resistance to cancer drugs, so it makes sense that it would play a role in breast cancer as well.

Importantly, the discovery of a potential connection between CD61 and this form of breast cancer may ultimately open up new avenues for treating this type of cancer more successfully.

“Detecting CD61 might help doctors determine what kind of therapeutic approach to use, knowing that they might be dealing with a more aggressive yet treatable form of breast cancer. For example, there are existing drugs that block CD61 signaling, which might be another potential aspect of treatment.”